1. Field of the Invention
The present invention relates in general to a microwave switch for controlling the transmission of microwave energy and a microstrip structure and, more particularly, to a planar solid state switching component which can be used at microwave frequencies and can be manufactured so that it is compatible with other planar monolithic gallium arsenide components.
2. Description of the Prior Art
Components currently in use as microwave switches are fabricated from silicon. These switches have been optimized to exhibit excellent performance characteristics. However, these devices are manufactured as discrete package components which must be inserted in hybrid microwave circuits. There are many current applications for which a microwave switch fabricated monolithically, i.e., on the same semiconductor wafer as the other microwave components in the circuit, would be more economical than if fabricated as a discrete component. Since microwave monolithic integrated circuits are fabricated from gallium arsenide, it is desirable to have a gallium arsenide microwave switch which is compatible with monolithic or planar fabrication processes.
Two different conventional component technologies have provided solutions to the problem of designing monolithic microwave switches. In one such technology, a gallium arsenide metal-semiconductor field effect transistor (MESFET) provides a microwave switch. A MESFET is described in U.S. Pat. No. 4,194,935. In a second technology, a gallium arsenide (GaAs) PIN diode has been formed as a monolithic microwave switch. The MESFET is limited in its power handling ability to about 1-3 watts. The gallium arsenide planar PIN diode can handle much more power but has a poor performance due to its large resistance when switched to the conducting state. This poor performance can be attributed to a fundamental physical property of gallium arsenide: It has an electronic band structure which is direct-gap and, consequently, has a very short excess carrier lifetime. This short lifetime, with the contributing effect of surface recombination due to the planar geometry, results in a degraded on-state resistance for the switch.
One of the basic shortcomings of conventional gallium arsenide microwave planar PIN diodes is that the intrinsic (I) or central region between the p- and n-regions has a relatively short time constant and unsatisfactory switching characteristics, even when the I region is formed of undoped gallium arsenide. Experience has shown that doping the I region of gallium arsenide tends to shorten the time constant even further, resulting in unsatisfactory operational characteristics of the switch.
Optical detectors have used the NIPI superlattice structure between p- and n-materials. The use of such a superlattice in a semiconductor is described in U.S. Pat. No. 4,695,857. U.S. Pat. No. 4,492,810 describes superlattice structures formed of GaAs. U.S. Pat. No. 4,411,728 describes a photodetector having a superlattice of a P-N periodic structure, with n.sup.+ impurities diffused or implanted on one of the opposite end faces perpendicular to the p-n junctions interface and a p.sup.+ impurity in the other end face. These structures were not, however, used in microwave switching and were designed to be activated optically. A solid state microwave switch is needed for controlling the transmission of microwave energy in a microstrip line structure.